1. Field of the Invention
The present invention relates generally to digital control circuits for radio frequency tuning systems, and, in particular, to digital control systems for tuning television receivers.
2. Description of the Related Art
Equipment for receiving radio-frequency signals contains control facilities by means of which a local oscillator needed for frequency conversion is set to the desired frequency. As a rule, use is made of the voltage-dependent capacitance of tuning diodes to vary the frequency of HF oscillators within wide limits. In most cases, two or more tuning diodes, whose control characteristics are paired to assure tracking at different tuning voltages, are used simultaneously.
In modern equipment, the manual adjustment of the tuning voltage is being replaced more and more by digital control circuits, whereby long-term stability and accuracy of adjustment and reproducibility are considerably increased. Such control circuits employ voltage or frequency synthesis methods, the latter using a phase-locked loop, which makes it possible to set precise, arbitrary local-oscillator frequencies.
A publication by ITT Semiconductors entitled "SAA 1290, Remote-Control and Tuning IC for TV Receivers," issue July 1986, Order No. 6251-222-5E, describes a monolithic integrated digital control circuit for tuning systems of television receivers which includes a facility for generating a tuning voltage by the voltage synthesis method. The output signal is a pulse-density-modulated pulse train ("PDM") which produces a PDM signal with an amplitude of 33 Volts using external switching devices. This signal is smoothed by a multisection low-pass filter to obtain the time-invariant tuning voltage. Accordingly, this facility for generating the tuning voltage is a dynamic digital-to-analog convertor which forms the desired high-resolution analog signal from the stored data by means of a pulse-density modulator.
The control circuit described includes additional dynamic digital-to-analog convertors which derive low-resolution analog function control voltages (e.g., for volume or brightness control) from stored data. It further includes a timing and control unit for the individual sequences of operations. The data stored under different addresses in the digital memory unit can be read in, fetched, or changed via a data input device, such as an externally accessible data bus. The address is, for example, the respective television channel number, which is activated via a remote-control circuit.
Offenlegungsschrift DE-A No. 34 27 852 (corresponding to U.S. Pat. Nos. 4,595,910 and 4,544,911) discloses a digital control circuit for tuning systems in television receivers. The disclosed digital control circuit uses a frequency synthesis method, such as a phase-locked loop. In that circuit, too, the tuning voltage is generated by means of a dynamic digital-to-analog convertor. The pulse-density modulator--called "digital mark/space-ratio controller" in the Offenlegungsschrift--is an accumulating counter whose count input is supplied with a data word to be added, and whose overflow output provides the PDM signal. On a time average within a given period, the overflow signal occurs in a ratio equal to the numerical ratio of the data word to be added and the number of possible counts.
A disadvantage of such analog or digital PLL control loops in tuning systems is, that to suppress oscillations and smooth the tuning voltage, they require a low-pass filter with a very low cutoff frequency or an integrator which slows down the change from one frequency to another. This is disadvantageous where a single tuner is to be switched rapidly between several signal sources (e.g., during multipicture reproduction in a television receiver or in a frequency-diversity receiving device in an automobile radio). If the switching takes too long, part of the information will be lost. On the other hand, the use of multituner designs in entertainment-electronics equipment is precluded because of their high cost.